1. Field of the Invention
This invention relates to a thread detecting apparatus in a sewing machine which is adapted to detect the break or exhaustion of a thread for sewing.
2. Description of the Prior Art
The conventional thread detecting apparatus includes a piezoelectric element made to contact mechanically with a thread passing through the needle hole of a throat plate. When the thread is still retained in a sewing work, the piezoelectric element, being rubbed upon by the the thread, generates an electric signal. On the other hand, when the thread is exhausted, the rubbing movement of the thread disappears and the piezoelectric element stops generating the electrical signal. The presence of the thread can be detected from the presence of this electrical signal. As such an apparatus, there is one which is disclosed, for example, in U.S. Pat. No. 4,429,651.
In the above mentioned apparatus, the thread is napped or injured on its surface and easily snaps off since the thread rubs upon the piezoelectric element.
In order to eliminate the above mentioned problem, it is possible to put a photoelectric element on an optical path used up intercepted by the thread and to know the presence of the thread by detecting if the thread intercepts the optical path or not.
According to this idea, however, the dust which is generated from threads and cloths in sewing work may often stay in the optical path and intercept the light through the path. Thus, the presence of the thread is detected by mistake even though the thread is really absent.